welding tig current source needs changed to constant voltage source

[kees54]

Hi All

I have need help how to change a schematic who is only constant current source smps for welding.

I have look some videos and hear that when use output capacitors there is a fase shift and so I need
a opamp to ivert it and outpu go to pwm chip V/feedback pin. The UC 3845 has such pin.

I have fount a schematic of a ordinary 3-60 volts 40 amp smps supply schematic who has both but this
is afcourse a very different use, I weld with it.

a resister voltage devider between smps output is used in video, then a inverter opamp (because it,s
output filter shift 180 degrees) the opamp has also a integrator on bord as error amplifier.

The UC3845 has a opamp build in who can be used so why some people use extern?

Maybe some here can make more clear how to do best make a welding mig smps inverter with constant
voltage, I do use the current feedback afcourse but then as protection agains overload.

thanks

kees

 

[kees54]

Never mind

I get it find out,just the UC3845 has a opamp inverter where can als be connected a integrator so
it act as a error amplifier.

I can just use a uc3845 circuit with also a current adjustment and I can weld tig/mig/stick. when
tig stick I remove the caps from output and have als there a working welder. I need however make
the feedback line clean with a cap outside welder connection.

I hope sy it right my english is not so technical advanced.

regards

 

[kees54]

Hi all

I have studie a lot about the kind of inverters for welding, nieuw types has high switch frequenties and fase shift zero voltage switching, etc.

I do now have the info to start but one thing I have not yet find out..

The inverter is for mig gas welder, it is after study needed to implemente a inductance setting what means I have to make the correction time of
responding on a short circuit adjustable, and the voltage HAS to be very stable when welding, so need a very good voltage feedback for this,
with respect afcourse of the current rise who can be quick (low inductance) to slow (much inductance/time) see picture and story i did get.

All the big weld companys use forward converters with a death time between switching up and lower leg for give trafo time to reset, did read that
also some dc rests can saturate trafo, it seems this is a very proven concept and it is simple and save otherwise kemppi

There was very much stuff, so much research there is going on.

here the tekst and the photo.

What does inductance do?

When MIG welding in the dip transfer mode, the welding electrode touches the weld pool, causing a short circuit. During the short circuit, the arc voltage is nearly zero. If the constant voltage power supply responded instantly, very high current would immediately begin to flow through the welding circuit. The rapid rise in current to a high value would melt the short-circuited electrode free with explosive force, dispelling the weld metal and causing considerable spatter.

Inductance is the property in an electrical circuit that slows down the rate of current rise (Figure 17). The current travelling through an inductance coil creates a magnetic field. This magnetic field creates a current in the welding circuit that is in opposition to the welding current. Increasing the inductance will also increase the arc time and decrease the frequency of short-circuiting.

For the delay in the current feedback I did think about a rc timing setup with a adjust pot, very simple will do.

kees

thanks for the help.